The present invention relates to a decoder for use in communication applications and, more particularly, to a decoder which will detect transmission of a preselected sequence of digit signals, which indicate that a particular communication device is being called.
In the past communication systems have been used in which each communication unit is assigned a multi digit code. When one unit is to be called by another unit, the code for the called unit will be transmitted, typically by the dual tone technique commonly used in telephone systems, and the called unit will detect transmission of its code number and provide an indication.
A common technique for detecting dual tone transmission of a preselected number sequence has been to use tapped inductors to detect the tones. The inductor taps are selected in a given sequence matching the number of the communication unit, with each set of taps being selected after the previous tone digit is decoded. The circuit will then await the next incoming tone digit and, if the proper sequence is received, will provide an indication that the unit has been called. One difficulty with such tap inductor decoders has been the relative bulk of the inductors needed for tone detection, miniaturization therefore has remained a difficult problem.
The Mostech Corporation, 1215 W. Crosby Road, Carrollton, Tex. 75006, has, however, recently developed a monolithic integrated circuit which detects the standard dual tone frequencies used in telephone dialing and which is extremely small and light. This integrated circuit tone receiver, MK5102N provides a four bit binary output which is indicative of the digit originally transmitted. In order to take advantage of this circuit, however, to detect when the desired preselected sequence of digits for an individual communication unit has been transmitted, an additional decoder circuit must be provided. Such a decoder circuit should be relatively small and inexpensive to construct.